Only 1 breast bud developed on dd..

Dd is only 8 (9 in June) and just before Christmas one breast began to develop.Now she has a prominent bud but only on one side, the other is completely flat.Anyone had experience of this? I'm sure mine grew at the same time. How long til we worry about the other one not growing?Tia

Well, I'm not entirely sure that this happened with my girls, but I've been reading a fair bit about puberty lately (looking up timescales of development) and it was mentioned somewhere that one breast bud is likely to develop first. So normal.